1. Field of the Invention
The present invention is directed to technology and methods for scanning an ultrasound beam in a free direction within a region of 3D space. The technology has particular applications within 3D medical ultrasound imaging, but also has applications in other areas of ultrasound imaging or other areas where mechanical movement of an object in 3D space is required.
2. Description of the Related Art
Three-dimensional (3D) ultrasound imaging of biological structures can be done by scanning a pulsed ultrasound beam in a 3D manner, and recording the back-scattered ultrasound signal in each beam direction. The principle was described already in the 50""ies, and several instruments have been built that applies the method.
With 3D imaging, it is important that the ultrasound beam is maximally focused, symmetrically in all directions around the beam axis, as one wants to observe the object from any direction (perspective), and small objects can be interrogated with a variety of beam directions. Such symmetric focusing can be obtained by subdividing the elements of a linear transducer array in the elevation direction, i.e. the direction normal to the two-dimensional (2D) scan plane. The 2D scan plane is often referred to as the azimuth scan plane, while the direction normal to the 2D scan plane is referred to as the elevation direction. Linear arrays with subdivision of the elements in the elevation direction are referred to as 1.5D or 1.75D arrays, depending on whether the elements can be symmetrically or individually steered around the azimuth mid plane. A problem with the subdivision of the elements is that the size of the elements are reduced, increasing the element impedance and hence also noise. The subdivision also gives a large total number of elements, leading to a large number of wires between the array elements and the electronic beam former. This increases cable losses and hence sensitivity of the array.
Symmetric focusing of the beam can be obtained with much fewer and larger elements with an array of concentric annular elements, the so-called annular array. The larger elements provide lower element impedances and hence less noise and cable losses. Another advantage with the annular array, is that it is covered in a dome, so that the array itself is not pushed against the body or other objects as the linear arrays typically are done. This allows the use of materials with lower characteristic impedance for mechanical backing support with the annular array compared to the linear arrays, giving less acoustic backing losses and hence better sensitivity with the annular array.
Hence the annular array provides better sensitivity for symmetrical focusing of the beam around the beam axis than the linear arrays, due to the large elements and low impedance backing material. The increased sensitivity in turn allows the use of higher ultrasound frequencies and hence better resolution for a given depth, than the linear arrays. The low number of elements also reduces the number of connecting cables and hence gives a lower manufacturing cost of the array.
A problem with the annular array for 3D beam scanning, is that scanning of the beam direction requires mechanical movement of the array. 3D scanning of the beam with small size of the whole scanning mechanism is then difficult, and the present invention provides a solution to this problem. With a particular embodiment of the invention, the annular array beam can be scanned in a 3D sector with more than 200 deg opening angle in both the elevation and the azimuth directions, with a dimension of the scan-head only slightly larger than the active radiation aperture of the array. The narrow head and wide scanning angle provide a probe that is small and easy to handle, and has special advantages for endoluminal and surgical imaging, where the probe is placed in narrow channels with limited possibilities to direction steer the probe, like for example with transvaginal, transrectal, transesophageal, transgastric, and transintestinal imaging, and imaging from narrow surgical wounds.